1. Field of the Invention
The present invention relates to a self-refresh apparatus and method for correcting self-refresh rate error which may be generated when a partial array self-refresh (hereinafter, referred to as “PASR”) operation is performed on a half of bank or a quarter of bank.
2. Description of the Background Art
A DRAM is used as a main memory device in most computer systems, whose cell requires a refresh operation to prevent loss of data.
A conventional semiconductor memory device performs a refresh operation on all cell arrays regardless of data storage in each cell, because an additional memory means for memorizing whether data are stored in each cell is required in the memory device.
As a result, a PASR operation is performed in order to solve the above-described problem.
The PASR operation may reduce power consumption because it is performed on a cell array having data during the same cycle.
For example, if a refresh operation is performed on only a half of a bank during a refresh rate, power consumption is reduced in half.
FIG. 1 is a diagram illustrating a self-refresh operation performed on cell arrays of a bank in a conventional memory 10 comprising four banks.
The memory cell arrays of a bank <2> are divided into two “HALF OF BANK”s (HOB) 15 and 16. Wordlines 11 and 12 are first and last wordlines of the “HALF OF BANK” 15. Wordlines 13 and 14 are first and last wordlines of the “HALF OF BANK” 16.
FIG. 2 is a timing diagram illustrating a self-refresh operation performed on “ALL BANK” for explaining a refresh rate. FIG. 3 is a timing diagram illustrating a self-refresh operation performed on “HALF of BANK” for explaining the refresh rate error.
When a refresh operation is performed on the bank <2>, an internal counter counts from wordline 11 to wordline 14. Here, a refresh rate is supposed to be 64 msec.
When an auto-refresh or self-refresh command (ALL BANK PASR) is performed on the whole bank <2>, the refresh operation time on each “HALF OF BANK” 15 and 16 is 32 m/sec. The refresh operation time on the whole bank <2> is 64 m/sec. That is, all wordlines 11˜14 should be consecutively activated in order to refresh all cells on the bank <2>. Here, it takes about 64 m/sec to re-activate the first wordline 11.
When the PASR operation is performed on a “HALF OF BANK”, the wordlines 11˜12 are consecutively activated for 64 msec.
According to a method of performing the PASR operation on the “half of bank” for 64 msec, the pulse interval of the refresh requiring signal is extended twice as much, and only internal addresses of cell arrays storing data are counted. Further, when a PASR operation is performed on a “QUARTER OF BANK” for 64 msec, the pulse interval of the refresh requiring signal is extended four times as much, and only internal address of cell arrays storing data are counted.
However, when the above-described self-refresh operation is performed on the “HALF OF BANK”, a self-refresh command on the “HALF OF BANK” is set as an EMRS (Extended Mode Register Set) code, and then the self-refresh operation is performed on the wordlines 11˜12, consecutively. After the self-refresh operation is performed on the wordline 12, the self-refresh operation is finished by an external command. Here, the self-refresh time performed on the wordlines 11˜12 is 64 msec.
As shown in FIGS. 2 and 3, the section “B” of FIG. 3 is twice longer than the section “A” of FIG. 2.
After the self-refresh operation is finished, an auto-refresh is performed pursuant to an auto-refresh command during the normal operation. Here, the internal address counter starts to count the wordline 13 in response to the auto-refresh command because the self-refresh operation is finished at the wordline 12.
That is, if the self-refresh operation is not finished, the internal address counter starts to recount the wordline 11. However, since the self-refresh operation is finished, the internal address counter starts to count the wordline 13 by a pulse interval “A” of FIG. 2.
It takes 32 msec to perform the auto-refresh operation consecutively from the wordline 13 to the wordline 14. As a result, it takes 96 msec to re-perform a refresh operation on the wordline 11 as shown in FIG. 3.
The refresh operation is performed on the wordline 11 after 96 msec although the refresh operation should be performed on each wordline per about 64 msec in a memory having a refresh rate of 64 msec, which may cause loss of data.